450 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



The dominant position which Lavoisier assigned to 

 oxygen is seen in the text preceding the table : 



' The acidifiable substances, combining with oxygen and 

 being converted into acids, acquire a great tendency to 

 combination ; they become capable of uniting with earthy 

 and metallic substances, and it is by this union that neutral 

 salts are produced. 



"This manner of viewing the acids does not permit me 

 to regard them as salts, although they have some of their 

 principal properties, such as solubility in water, etc. . . . 

 For the same reason, I shall no longer place the alkalis nor 

 the earthy substances, such as lime, magnesia, etc., in the 

 class of salts, and I shall designate by this name only 

 compounds formed by the union of an oxidised element 

 and a base" ( Works, I. 115-116). 



Lavoisier's broad classification of the elements into three 

 groups, (i) me.tals, (2) inflammable elements, yielding acids 

 by combustion, and (3) oxygen, the essential constituent of 

 acids, bases and salts, is curiously similar to the alchemistic 

 system in which the three elements were (i) mercury, the 

 typical metal, (2) sulphur, the typical combustible substance, 

 and (3) salt. 



Oxygen and chlorine as "supporters of combustion 

 (Davy, 1812). Davy, in his Elements of Chemical Philo- 

 sophy (1812), followed Lavoisier's classification but made 

 certain important modifications. His three groups were : 



(i) Elements that support combustion, oxygen, and 

 chlorine, which Davy had just shown to be an element. 



prove this experimentally. The alkalis were omitted because Lavoisier 

 regarded them as compounds, in view of Berthollet's proof of the com- 

 posite nature of ammonia. Silica is not mentioned in the text accom- 

 panying the table, but seems to have been classified with the earths, 

 rather than with the boric radical, because it was insoluble in water. 



2 Arranged in alphabetical order in French. In the text they are 

 described, by an obvious misprint, as "acidifiable" instead of "salifi- 

 able." 



3 Three unknown elements, assumed to be present as oxides in 

 muriatic, " fluoric," and boric acids. 



